How can I convert a square wave output to a nearly sine wave output?

Syncopator said:

Yes, it's quite easy.
I would recommend using a Wien bridge oscillator. You can find many examples on the web, using a variety of amplitude stabilising methods. I suggest you have a search so you can decide which method you want to use.

Here's one version which I designed. The amplitude control may be rather more complicated than you want, but it works well.





The frequency of a Wien oscillator is given by the following equation



A value of 120k for the two Rs and 27nF for the two Cs give a calculated frequency of 49Hz. You may have to make small adjustments, as necessary, to get the frequency exact - if it's important.

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I used the same method which you mentioned to make a 50Hz 220V power supply for my record turntable when I was living in South America, where the local mains was about 60Hz and somewhere around 110V. Neither of those figures being reliable. I did actually start with a special chip which gave a precise 50Hz out. It was derived from a crystal oscillator with a suitable dividers in the chip. The output was a square wave, which I passed through a band pass filter.
The sine wave drove a 4 -5 watt amplifer which had a reversed mains transformer at its output.
I don't think the chip is manufactured any more. But that's irrelevant

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goldsmith said:

Dear baghu
Hi
I'm agree with syncopator . it seems good way .
There are many many ways to obtain your aim .
If you tell me a bit more explanation , that how much quality , and price do you want , i can suggest you best way to achieve your purpose .
You can use linear ways , or switch mode ways . both of them are easy , but at switch mode , systems , you'll be able to decrease the price and increase the quality .
BTW : one of simple ways , that you can do is royer oscillator ( but it's out pu is square wave ) . and it is not good way for you , i think.
if you need powers up to some KW , the switch mode ways are best choices ( as you told , you want 50 W , but if you want more powers , no problem )
And you can do it with your self
All the best
Goldsmith

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keith1200rs said:

I posted a slightly simpler circuit not so long ago. The sine wave will not be quite as pure as the JFET controlled version.

https://www.edaboard.com/threads/241017/#post1032292

Keith.

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baghu said:

Thank youfor the thread, Keith. What frequency of approximate sinewave does this circuit generate? What amplifier do I need? Thanks.

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baghu said:

I am sorry to say that ... , I can't understand where the output comes out. Also I am unsure about the IC. I shall be very grateful if you kindly elaborate.

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Syncopator said:

The i.c. at the top is the oscillator. The one beneath it is part of the amplitude control. The output from the oscillator is buffered by the voltage follower at the right hand side. It has a potentiomoter at its input so that the amplitude applied to the subsequent circuit/s may be adjusted as desired.

I tried three types of i.c., LF351, TL071 and 5534, simply because I had them among my components. They all worked satisfactorily. More or less any commonly available op-amp will work too.

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